Conventionally, there are control devices that perform control of the number of pumps in reference to a predetermined parameter as a pump degradation factor. For example, in Japanese Patent Unexamined Publication 2009-133253, the time of operation of each of the pumps is set in advance, and operation/stop of a plurality of pumps are determined according to whether or not the cumulative pump operation time reaches the set time of operation. The cumulative operation times of the respective pumps are thereby equalized.